In the production of corrugated cardboard two corrugated rollers are used, in charge of making the corrugated conformation of the intermediate paper or core of the cardboard, and a flat roller, in charge of fixing another paper, which determines one of the flat faces of the cardboard, on the corrugated one. The paper building up the other flat face of the cardboard is glued on later in another part of the installation.
The following factors intervene in the corrugation of the intermediate paper and the gluing of the flat paper on it.
Pressure: the rollers exercise a pressure of between 3 and 4 kg/mm2 for the corrugation and a pressure of between 2 and 3 kg/mm2 for the gluing.
Temperature: being it required that the paper reaches a temperature of about 180xc2x0 C. for which the rollers are warmed by introducing saturate vapour in its interior.
Humidity: the paper should have certain humidity, for the corrugation formation and the gluing to be correct.
Therefore, in the mentioned production of corrugated cardboard two corrugated rollers and a flat one are made to rotate, exercising high pressure between them, which causes that the corrugating device and the mentioned rollers support a high vibration level while in operation, mainly the flat roller, as it has to rotate against a notched roller.
The mentioned vibrations have two fundamental features to be kept in mind:
Vibration frequency: which depends on the notch pitch and the rotation speed of the rollers.
Vibration level: which is determined by very diverse factors, such as for example the structure of the machine, foundations, inertia and elasticity of the rollers, the profile type of the notch, the state of the rollers, the thickness of the used sheets of paper, the corrugating and gluing forces, as well as the production speed, or which is the same as the vibration frequency.
In a corrugating device with certain rollers, the only variable to change the vibration frequency is the rotation speed of the rollers, but this is defined by the type and quantity of orders, so that actuating on this variable is very restricted, or even null.
On the other hand, it is the vibration level which most affects the problems and it can happen during the production of corrugated cardboard, as when this level is excessively high, too strong marks appear and even cuttings in the produced cardboard.
Among the factors which affect the vibration level there is the design of the corrugating device, so that depending on this corrugating design there are some with more or less propensity to have vibration problems. Another factor in this respect is the vibration frequency, which in principle can be any and therefore can coincide with some of the resonance frequencies (or natural frequencies) of some part of the corrugating device, as for example the rollers; so that when the corrugating device is especially problematic, low weight sheets are used, i.e. they provide little damping, the notch pitch profile is high and at great height, if moreover the resonance frequencies of the rollers are reached, the possibilities of the cardboard to be faulty are very high at certain speeds.
To solve the problem solutions have been attempted, such as reducing the corrugation and gluing forces to the maximum so as to reduce in consequence the vibration level and the mark of the cardboard but this solution brings about other inconveniences, such as the lack of forces at other speeds different from the resonance one (especially at high speeds), as well as the unstableness of the corrugating device, which is seen in the paper connections.
Another purpose to solve the problem is by trying to move away the first natural frequency from the frequency ranges of the operation vibration, but nevertheless the geometric and material limitations of the roller design make that the frequency deviation possibility is not enough, always having it inside the working ranges.
The first natural frequency of the rollers is the only one that is reached in the corrugating devices, since when the vibration frequency is double the first frequency, it also excites the roller to the first natural frequency. The first vibration mode represents the form that the roller acquires when this gets excited at the first natural frequency, being this form similar to that of roller deformation, when it flexes by a load applied in the center, i.e. that the deformation is maximum in the center and it falls progressively towards the ends. From this it can be seen that the problems of an excess mark and cardboard cutting in the first natural frequency appears in the central area of the rollers with tendency to disappear towards the end.
According to the present invention a one solution is proposed to the mentioned vibration problem with the corrugating devices for the production of corrugated cardboard, based on a damping system applied to the rollers of the mentioned corrugating devices, by means of which the vibration energy is absorbed reducing this way its level.
This system object of the invention consists in including inside the rollers a damping device in solidary assembly respect to the wall of the corresponding roller, so that when vibrations take place inside the roller, the interior device also vibrates being opposed its vibration to that from the activation source, which is the roller, so that the vibrations of this are damped, disappearing the negative causes of the same on the paper to produce the corrugated cardboard.
The damping effect takes place especially at critical speeds, which is when the roller resonance is reached; being foreseen a realization of the device that acts particular at these speeds, while another realization damps the vibrations at all the critical speeds, optimizing the damping at these speeds.
The assembly of the device respect to the roller wall is established by means of a flexible jacket and with interference adjustment, so that the dilation differences because of the temperature are absorbed without affecting the roller outside.
The adjustment areas between the damping device and the roller wall are moreover established with holes which allow the vapour and condensate passage from one side to the other, while centrally through the damping device itself a conduit is defined which allows the vapour passage from the entrance tube, with which the roller heating does not suffer damage because of the damping device incorporation.
The preconized system provides an advantageous practical solution against the vibration problem of the mentioned rollers to produce corrugated cardboard, fulfilling with the following particularities:
The damping device can be designed easily to work at the heating temperature of the application rollers and in the atmosphere of saturated vapour corresponding to this heating, without deformation or corrosion damage.
In its installation the damping device does not affect the heat transfer inside the corresponding rollers, so that the heating of these does not suffer alteration.
The evolution of the behaviour of the damping device in time is not significant during the useful life of the roller, resulting in no maintenance.
The temperature differences that can be obtained, to avoid that the dilations of some areas regarding the others produce deformations in the rollers, do not affect the operation.
The damping device is installed in the vapour chamber determined inside the rollers, and its incorporation supposes no nuisance whatsoever to the conformation process of the corrugated cardboard, so that no change in the installation is necessary.